INFLUENCE OF PHYSICAL AGENTS. 161 
rature, in the open air where all were exposed, was 11° C. All of the 
spores failed to grow after an exposure of fourhours. When exposed 
in water the time of exposure was longer. 
Roux has shown that the light also has an effect upon the culture 
medium, and that sterilized bouillon which has been exposed to direct 
sunlight for some hours restrains the development of anthrax spores 
subsequently introduced into it, but not of the growing bacilli. His 
experiments show that access of oxygen is a necessary factor in the 
sterilization of cultures by sunlight. 
In the experiments of Momont (1892) dry anthrax spores were 
found to resist the action of light for a long time, but moist spores, 
freely exposed to the air, failed to grow after forty-four hours’ ex- 
posure to sunlight. In the absence of spores, anthrax bacilli in a 
moist condition, when freely exposed to the air, failed to grow after 
exposure to sunlight for half an hour to two hours; but in the ab- 
sence of air the same bacilli were not destroyed at the end of fifty 
hours’ exposure. 
Geisler (1892), in experiments made upon the typhoid bacillus, 
found that all portions of the solar spectrum except the red rays ex- 
ercised a restraining influence upon the development of this bacillus. 
The electric light gave a similar result. The most decided effect was 
produced by rays from the violet end of the spectrum. The restrain- 
ing influence appears, from the researches of Geisler, not to be due 
solely to the direct action of light upon the development of the 
bacilli, but also to changes induced in the gelatin culture medium 
employed in his experiments. 
In his address before the International Medical Congress of Berlin, 
1890, Koch states that the tubercle bacillus is killed by the action of 
direct sunlight in a time varying from a few minutes to several hours, 
depending upon the thickness of the layer exposed. Diffused day- 
light also has the same effect, although a considerably longer time of 
exposure is required—when placed close to a window, from five to 
seven days. 
Dieudonné (1894), in experiments upon Bacillus prodigiosus and 
Bacillus fluorescens putidus, found that direct sunlight in March, 
July, and August killed these bacilli in one and one-half hours, in 
November in two and one-half hours. Diffuse daylight in March 
and July restrained development after three and one-half hours’ ex- 
posure (in November four and one-half hours), and completely de- 
stroyed vitality in from five to six hours. 
Ward’s experiments (1892-1894) show that the blue and violet 
rays have decided germicidal power, while the rays at the red end of 
the spectrum are comparatively inert. This corresponds with results 
previously reported by Arloing. 
11 
